Pisgah Astronomical Research Institute
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pisgah Astronomical Research Institute clear sky chart from 2005-11-06 to 2025-08-05.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	13% 6% 2 6% 5% 5% 5% 4 6% 7% 40%	125622
	Night	15% 6% 2 5% 4 4 4 4 5% 7% 45%	122451
Transparency	Day	39% 9% 11% 18% 16% 7%	129346
	Night	37% 7% 11% 18% 18% 8%	126796
Seeing	Day	20% 15% 24% 21% 14% 7%	16000
	Night	19% 12% 21% 26% 19% 4	26040

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	12% 5% 1 4 2 3 3 2 5% 5% 58%	4056
	11th to 20th	Day	10% 5% 2 6% 3 2 2 2 3 3 61%	4060
	21st to end	Day	17% 5% 2 6% 4 3 3 2 4 5% 49%	4466
February	1st to 10th	Day	14% 6% 3 6% 4 4 3 3 6% 6% 45%	1925
	11th to 20th	Day	18% 8% 2 5% 4 4 4 3 4 5% 43%	1948
	21st to end	Day	17% 7% 1 6% 5% 5% 3 4 5% 5% 42%	1650
March	1st to 10th	Day	20% 6% 2 6% 4 3 4 3 5% 5% 42%	4794
	11th to 20th	Day	12% 6% 2 4 4 3 3 2 4 6% 54%	4980
	21st to end	Day	18% 6% 2 5% 3 4 3 3 5% 5% 45%	5465
April	1st to 10th	Day	12% 6% 2 6% 4 3 4 5% 7% 8% 43%	2200
	11th to 20th	Day	14% 6% 2 6% 4 3 5% 4 6% 7% 43%	2388
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 5% 7% 46%	2600
May	1st to 10th	Day	13% 8% 3 9% 5% 5% 6% 4 6% 7% 34%	5862
	11th to 20th	Day	19% 7% 3 7% 5% 5% 6% 5% 7% 8% 28%	5861
	21st to end	Day	14% 6% 3 6% 4 5% 6% 4 7% 8% 38%	6433
June	1st to 10th	Day	12% 6% 3 7% 6% 6% 7% 5% 8% 10% 29%	2600
	11th to 20th	Day	7% 6% 2 6% 6% 7% 7% 7% 9% 10% 32%	2600
	21st to end	Day	8% 7% 2 7% 7% 7% 6% 6% 8% 10% 33%	2600
July	1st to 10th	Day	8% 5% 2 7% 7% 7% 7% 6% 8% 11% 34%	5900
	11th to 20th	Day	9% 5% 3 9% 7% 8% 7% 6% 8% 11% 26%	5900
	21st to end	Day	11% 7% 4 11% 8% 8% 8% 7% 8% 9% 20%	6424
August	1st to 10th	Day	12% 7% 3 9% 6% 7% 8% 7% 8% 10% 24%	4156
	11th to 20th	Day	11% 5% 2 8% 6% 6% 6% 5% 8% 11% 31%	4044
	21st to end	Day	8% 4 2 7% 5% 6% 6% 5% 6% 8% 42%	4150
September	1st to 10th	Day	12% 7% 3 8% 5% 5% 6% 5% 7% 8% 33%	2090
	11th to 20th	Day	17% 8% 3 7% 5% 5% 6% 5% 6% 7% 31%	2090
	21st to end	Day	18% 6% 2 6% 5% 5% 4 3 6% 7% 38%	2090
October	1st to 10th	Day	15% 5% 2 4 3 4 3 3 5% 6% 49%	2079
	11th to 20th	Day	12% 5% 2 3 2 3 3 3 5% 5% 57%	1900
	21st to end	Day	15% 4 1 4 3 2 3 3 4 5% 56%	2090
November	1st to 10th	Day	12% 7% 2 4 2 2 3 2 4 6% 55%	1852
	11th to 20th	Day	13% 6% 1 4 4 3 3 2 4 6% 54%	1800
	21st to end	Day	14% 4 2 4 3 3 3 3 6% 6% 51%	1800
December	1st to 10th	Day	14% 6% 2 6% 3 3 3 3 4 4 52%	2930
	11th to 20th	Day	15% 6% 2 5% 4 4 4 4 5% 6% 45%	3373
	21st to end	Day	21% 4 1 4 2 3 3 2 4 6% 48%	4466
January	1st to 10th	Night	14% 4 1 3 2 3 3 2 3 5% 59%	5889
	11th to 20th	Night	8% 5% 2 6% 4 4 4 3 4 5% 56%	5900
	21st to end	Night	17% 5% 1 3 3 3 4 3 5% 6% 51%	6490
February	1st to 10th	Night	13% 6% 2 5% 5% 4 4 3 5% 6% 48%	2473
	11th to 20th	Night	17% 7% 2 5% 4 3 4 3 4 5% 45%	2338
	21st to end	Night	16% 7% 2 5% 3 4 4 4 5% 7% 42%	1980
March	1st to 10th	Night	19% 8% 2 6% 4 4 3 3 5% 5% 42%	5057
	11th to 20th	Night	12% 4 1 3 3 3 4 3 5% 8% 54%	4980
	21st to end	Night	18% 5% 2 5% 3 4 4 3 5% 6% 46%	5467
April	1st to 10th	Night	15% 7% 2 5% 2 3 3 3 5% 7% 49%	2200
	11th to 20th	Night	14% 5% 2 5% 3 2 2 3 5% 6% 52%	1973
	21st to end	Night	13% 5% 2 4 4 3 4 3 4 7% 51%	1800
May	1st to 10th	Night	12% 8% 2 6% 4 5% 5% 5% 6% 9% 36%	4033
	11th to 20th	Night	23% 7% 3 6% 4 4 5% 4 6% 8% 31%	4032
	21st to end	Night	15% 5% 2 5% 4 4 4 4 6% 9% 42%	4421
June	1st to 10th	Night	11% 7% 4 6% 5% 4 5% 6% 9% 10% 33%	1800
	11th to 20th	Night	9% 6% 2 7% 5% 4 5% 4 8% 9% 41%	1800
	21st to end	Night	8% 5% 2 5% 4 4 7% 4 8% 11% 41%	1800
July	1st to 10th	Night	9% 5% 2 8% 6% 5% 7% 6% 8% 10% 34%	4058
	11th to 20th	Night	10% 6% 2 7% 6% 5% 7% 6% 8% 12% 32%	4057
	21st to end	Night	12% 8% 3 9% 6% 6% 6% 5% 7% 10% 28%	4427
August	1st to 10th	Night	13% 8% 3 8% 6% 6% 6% 4 7% 9% 32%	2861
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 4 7% 7% 33%	2785
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 9% 46%	3469
September	1st to 10th	Night	13% 7% 2 7% 4 5% 6% 5% 7% 9% 37%	2090
	11th to 20th	Night	20% 7% 2 7% 5% 4 4 4 6% 7% 34%	2090
	21st to end	Night	19% 7% 2 6% 4 4 4 2 4 6% 43%	2090
October	1st to 10th	Night	15% 6% 2 4 3 3 4 4 4 7% 49%	2099
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 5% 53%	2280
	21st to end	Night	15% 5% 1 3 3 3 3 2 3 4 57%	2508
November	1st to 10th	Night	15% 6% 2 3 3 2 4 3 4 5% 53%	2442
	11th to 20th	Night	15% 6% 1 4 3 2 3 3 4 6% 53%	2600
	21st to end	Night	15% 4 2 4 4 3 4 3 5% 6% 50%	2600
December	1st to 10th	Night	12% 6% 3 5% 4 4 3 3 4 4 52%	4250
	11th to 20th	Night	17% 6% 2 5% 4 4 4 3 4 5% 46%	4822
	21st to end	Night	22% 6% 2 4 3 3 3 3 4 6% 44%	6490

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	29% 4 5% 15% 36% 12%	4057
	11th to 20th	Day	30% 3 4 12% 35% 16%	4060
	21st to end	Day	38% 4 5% 15% 24% 15%	4466
February	1st to 10th	Day	39% 6% 8% 20% 19% 8%	1925
	11th to 20th	Day	43% 5% 8% 16% 20% 7%	1958
	21st to end	Day	44% 6% 7% 15% 20% 9%	1650
March	1st to 10th	Day	43% 3 8% 14% 18% 14%	4794
	11th to 20th	Day	32% 4 5% 14% 24% 20%	4980
	21st to end	Day	40% 5% 7% 26% 14% 6%	5465
April	1st to 10th	Day	37% 7% 11% 21% 19% 6%	2200
	11th to 20th	Day	38% 8% 9% 18% 19% 7%	2388
	21st to end	Day	36% 9% 12% 25% 16% 3	2600
May	1st to 10th	Day	46% 10% 13% 18% 11% 2	5897
	11th to 20th	Day	50% 11% 14% 16% 9% 1	5865
	21st to end	Day	39% 11% 13% 19% 16% 3	6447
June	1st to 10th	Day	45% 12% 17% 18% 6% 1	2600
	11th to 20th	Day	40% 14% 19% 17% 9% 1	2600
	21st to end	Day	41% 13% 16% 18% 10% 2	2600
July	1st to 10th	Day	40% 14% 18% 21% 7%	5835
	11th to 20th	Day	45% 16% 25% 11% 2 1	5831
	21st to end	Day	55% 14% 14% 12% 5%	6240
August	1st to 10th	Day	48% 13% 17% 18% 3	4143
	11th to 20th	Day	40% 13% 23% 12% 8% 3	4083
	21st to end	Day	35% 11% 14% 27% 12% 1	4150
September	1st to 10th	Day	44% 12% 11% 17% 14% 3	2090
	11th to 20th	Day	49% 10% 10% 16% 14% 2	2090
	21st to end	Day	44% 8% 11% 16% 15% 6%	2090
October	1st to 10th	Day	28% 4 8% 18% 31% 11%	3456
	11th to 20th	Day	20% 4 5% 17% 26% 28%	3140
	21st to end	Day	29% 5% 6% 21% 22% 17%	3428
November	1st to 10th	Day	31% 6% 6% 19% 27% 12%	1852
	11th to 20th	Day	32% 4 5% 21% 26% 11%	1800
	21st to end	Day	34% 5% 7% 19% 27% 8%	1800
December	1st to 10th	Day	36% 5% 5% 16% 25% 14%	2930
	11th to 20th	Day	39% 5% 8% 16% 19% 13%	3370
	21st to end	Day	36% 5% 10% 26% 18% 5%	4466
January	1st to 10th	Night	30% 4 7% 14% 28% 18%	5889
	11th to 20th	Night	31% 5% 6% 11% 31% 16%	5900
	21st to end	Night	35% 4 8% 14% 24% 15%	6490
February	1st to 10th	Night	37% 5% 10% 20% 20% 8%	2473
	11th to 20th	Night	42% 5% 7% 17% 20% 7%	2339
	21st to end	Night	41% 7% 8% 15% 21% 9%	1980
March	1st to 10th	Night	41% 6% 8% 13% 24% 8%	5057
	11th to 20th	Night	28% 6% 7% 18% 22% 19%	4980
	21st to end	Night	39% 6% 9% 24% 15% 8%	5467
April	1st to 10th	Night	35% 6% 11% 21% 23% 4	2200
	11th to 20th	Night	33% 5% 10% 21% 24% 7%	1973
	21st to end	Night	33% 8% 15% 25% 18% 2	1800
May	1st to 10th	Night	43% 11% 13% 23% 9% 1	4060
	11th to 20th	Night	48% 11% 17% 16% 8%	4033
	21st to end	Night	35% 11% 17% 24% 12% 1	4429
June	1st to 10th	Night	40% 13% 18% 21% 7% 1	1800
	11th to 20th	Night	35% 11% 23% 22% 8% 1	1800
	21st to end	Night	34% 13% 20% 19% 12% 3	1800
July	1st to 10th	Night	40% 14% 20% 21% 5%	4015
	11th to 20th	Night	40% 17% 27% 14% 2 1	4006
	21st to end	Night	48% 14% 18% 15% 5%	4297
August	1st to 10th	Night	45% 11% 24% 16% 5%	2850
	11th to 20th	Night	46% 13% 17% 13% 10% 1	2812
	21st to end	Night	33% 9% 18% 29% 10% 1	3469
September	1st to 10th	Night	41% 11% 14% 18% 12% 3	2090
	11th to 20th	Night	46% 11% 11% 17% 13% 1	2090
	21st to end	Night	42% 5% 10% 21% 16% 5%	2090
October	1st to 10th	Night	28% 6% 12% 16% 29% 9%	3502
	11th to 20th	Night	21% 4 5% 17% 28% 25%	3779
	21st to end	Night	32% 3 4 19% 25% 16%	4120
November	1st to 10th	Night	34% 5% 8% 18% 23% 12%	2442
	11th to 20th	Night	34% 4 6% 17% 26% 13%	2600
	21st to end	Night	36% 5% 5% 19% 25% 9%	2600
December	1st to 10th	Night	35% 5% 6% 18% 24% 12%	4250
	11th to 20th	Night	41% 6% 7% 16% 19% 12%	4824
	21st to end	Night	40% 5% 11% 18% 21% 5%	6490

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	29% 27% 32% 11% 1	300
	11th to 20th	Day	30% 30% 26% 12% 2	300
	21st to end	Day	30% 33% 30% 5% 2 1	330
February	1st to 10th	Day	26% 28% 36% 9% 1	300
	11th to 20th	Day	40% 29% 26% 5%	300
	21st to end	Day	39% 34% 20% 6% 1	249
March	1st to 10th	Day	28% 24% 27% 15% 6%	432
	11th to 20th	Day	27% 25% 29% 15% 3 1	500
	21st to end	Day	29% 25% 27% 14% 5% 1	550
April	1st to 10th	Day	26% 20% 26% 19% 8% 1	500
	11th to 20th	Day	18% 24% 28% 19% 9% 1	492
	21st to end	Day	21% 17% 26% 22% 11% 4	500
May	1st to 10th	Day	19% 20% 31% 16% 9% 5%	503
	11th to 20th	Day	22% 12% 24% 23% 12% 7%	527
	21st to end	Day	21% 10% 19% 26% 14% 10%	573
June	1st to 10th	Day	15% 8% 23% 28% 15% 10%	530
	11th to 20th	Day	11% 7% 22% 24% 22% 13%	530
	21st to end	Day	10% 6% 24% 31% 17% 13%	530
July	1st to 10th	Day	10% 7% 15% 25% 27% 17%	525
	11th to 20th	Day	10% 3 11% 28% 27% 21%	524
	21st to end	Day	10% 4 13% 27% 26% 20%	576
August	1st to 10th	Day	21% 6% 16% 22% 19% 17%	502
	11th to 20th	Day	13% 5% 15% 28% 24% 14%	479
	21st to end	Day	14% 2 12% 28% 29% 15%	539
September	1st to 10th	Day	11% 7% 14% 30% 28% 11%	490
	11th to 20th	Day	20% 5% 15% 26% 25% 10%	490
	21st to end	Day	22% 8% 16% 30% 19% 5%	490
October	1st to 10th	Day	20% 7% 22% 25% 18% 8%	490
	11th to 20th	Day	13% 15% 26% 27% 15% 4	490
	21st to end	Day	19% 11% 28% 29% 10% 3	539
November	1st to 10th	Day	21% 15% 29% 22% 12% 2	390
	11th to 20th	Day	23% 19% 34% 20% 4	300
	21st to end	Day	17% 19% 43% 15% 5% 1	300
December	1st to 10th	Day	30% 28% 29% 11% 2	300
	11th to 20th	Day	24% 20% 40% 13% 4	300
	21st to end	Day	32% 18% 34% 12% 4 1	330
January	1st to 10th	Night	23% 20% 30% 22% 5%	800
	11th to 20th	Night	21% 22% 31% 21% 6%	800
	21st to end	Night	20% 21% 34% 23% 3	880
February	1st to 10th	Night	17% 21% 29% 30% 4	800
	11th to 20th	Night	24% 21% 32% 21% 2	783
	21st to end	Night	24% 25% 27% 21% 3	660
March	1st to 10th	Night	21% 19% 29% 24% 6%	800
	11th to 20th	Night	20% 18% 30% 23% 8% 1	800
	21st to end	Night	21% 18% 29% 22% 9%	880
April	1st to 10th	Night	21% 20% 26% 24% 9% 1	800
	11th to 20th	Night	21% 14% 25% 25% 14% 2	668
	21st to end	Night	15% 11% 23% 31% 19% 2	600
May	1st to 10th	Night	16% 14% 26% 24% 19% 2	600
	11th to 20th	Night	21% 7% 20% 27% 23% 3	591
	21st to end	Night	17% 5% 16% 24% 31% 8%	650
June	1st to 10th	Night	14% 4 14% 32% 29% 7%	600
	11th to 20th	Night	11% 3 15% 26% 34% 12%	600
	21st to end	Night	9% 4 14% 27% 39% 8%	600
July	1st to 10th	Night	12% 2 9% 24% 43% 10%	585
	11th to 20th	Night	12% 2 7% 24% 43% 13%	582
	21st to end	Night	8% 4 7% 28% 40% 13%	657
August	1st to 10th	Night	16% 3 10% 21% 39% 12%	579
	11th to 20th	Night	17% 4 8% 24% 37% 9%	564
	21st to end	Night	14% 4 7% 23% 41% 11%	665
September	1st to 10th	Night	15% 4 10% 26% 36% 9%	760
	11th to 20th	Night	24% 4 11% 22% 32% 7%	760
	21st to end	Night	22% 5% 13% 30% 26% 4	760
October	1st to 10th	Night	17% 8% 18% 24% 24% 8%	760
	11th to 20th	Night	18% 11% 18% 29% 20% 3	760
	21st to end	Night	19% 10% 18% 33% 17% 3	836
November	1st to 10th	Night	18% 8% 20% 32% 19% 3	780
	11th to 20th	Night	18% 13% 23% 32% 11% 2	800
	21st to end	Night	18% 14% 23% 30% 14% 2	800
December	1st to 10th	Night	23% 17% 26% 27% 8%	800
	11th to 20th	Night	20% 14% 27% 31% 8% 1	800
	21st to end	Night	28% 14% 24% 27% 6%	880

Page generated at 2025-08-02 01:32:25. Version ac07

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